(a) Color fringing and other sources of excess color in black-and-white image elements--In the copying of color documents that contain black-and-white material such as text, a color fringe commonly appears in the transitions between black and white, at each edge (or at least many edges) within that monochrome material. "Color fringe" here means chromatic color that was not present in an original, and that usually is introduced into an image by tiny misalignments of the red, green and blue additive-primary detection systems in a scanning stage.
(b) Throughput problems, and perception--It is undesirable to leave this color fringe, or what may be called "excess color", in the image data. Because of the way most color printers work, black-and-white text with even the smallest amount of color cannot be printed as quickly as the same text which contains absolutely no color. A very significant difference in printing speed results.
It is helpful to understand that the problem addressed here is not one of visual or esthetic defect. Usually the color fringe is only on the order of one or a very few pixels, which the unaided human eye ordinarily cannot readily resolve--at least not in the presence of an intimately associated high-contrast transition, namely the black-white transition which generated the fringe.
Thus the problem is purely one of printing throughput (e.g., pages per unit time). When pure black-and-white material is to be printed, a system can use exclusively black colorant and just a single pass.
Even one color dot in a line of black-and-white text, however, is enough to greatly impact the printing process. It requires the system to print in multiple, overlapping passes--and using printheads that dispense chromatic colorant.
If black-and-white matter has been previously detected, the printer could be simply instructed to ignore the color dots. This, however, would form a major disruption of the printing protocols in the system. A faster and simpler cure is desired.
Analogously, the scanner cannot simply be set to monochrome, for the black-and-white material as indicated above is embedded within a color image. A user could be required to point out the black-and-white material to the system in advance, but preferably operation in this regard should be entirely automatic.
(c) Related efforts to resolve the problem--It is known in commercial products to establish a subsystem that watches for certain kinds of objects, black-and-white text in particular, and upon identifying such an object removes associated chromatic-color data. Text may be identified in various ways, as for example in the previously mentioned application of Noh.
Subsystems for identifying and then removing excess color take a variety of forms, and in fact remove very nearly all the color--but unfortunately all are imperfect in end result. Such failure to remove all color often arises from interaction between some minor feature in text and a detecting protocol of the subsystem.
For instance, underscoring--or other shallow or small structures, or spatially extended structures--diverge from the characteristics of text characters that are commonly employed in text recognition. They thereby confuse the recognition protocols.
Another problem that can complicate the issues is the desirability of looking at nearby associated areas of the image to determine whether a particular region of interest is likely to be black-and-white text. Such examination when performed in immediate conjunction with text recognition becomes unduly complex.
Furthermore, sometimes removing excess color in a region--on the basis that that area and also nearby areas seem to contain text--can be counterproductive. Such tests have a tendency to logically interconnect areas in which standard decisional protocols may be wrong, and so may result in propagating incorrect decisions. This type of error can result in removing macroscopic chromatic-color features that are significant in the image--making untrue the suggestion earlier in this document that fringing does not produce perceptible image defects.
(d) Conclusion--Color fringing, and resultant low throughput in black-and-white text regions, have continued to impede achievement of uniformly excellent inkjet printing--at high throughput--on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.